1. Field of Invention
The present invention generally relates to semiconductor processing and, more particularly, to a method and system for endpoint detection of plasma chamber cleaning processes and/or etch processes.
2. Related Art
Plasma etching and deposition is now a common process in semiconductor manufacturing to form active areas, lines, holes, and other device features. During plasma etching and/or deposition, process gases are introduced into a plasma chamber where they are excited by power sources to form a plasma. The plasma species react with exposed substrate areas and form volatile species which depart from the substrate, thereby forming desired features of a semiconductor device. The plasma then becomes a complex mixture of native reactant gases and etched products, including molecules, ions, neutral radicals, and other species.
Plasma processing of wafers within a plasma chamber has an effect on the interior of the processing chamber. Reaction by-products can deposit on the inside surface of the chamber walls and cause serious problems as they accumulate. For example, in addition to affecting the chamber processing conditions, thereby degrading the processing stability, the accumulated by-products on the chamber walls may dislodge and settle on the surface of the substrate causing defects that are likely to negatively affect the entire chip. Therefore, the removal of the accumulated by-products becomes extremely important.
Processing chambers are typically removed from the production line on a scheduled, periodic basis for a cleaning operation to address the above-noted conditions. Cleaning operations can include plasma cleans of the interior of the chamber, wet cleans of the interior of the chamber, and replacement of certain components of the processing chamber which may actually be consumed by the plasma processes conducted therein.
In-situ plasma cleaning is desirable due to its efficiency. During plasma cleaning, cleaning gases are introduced into the chamber to react with the by-products accumulated on the chamber walls and to reduce the thickness thereof by chemical action, mechanical action, or both. Resulting vapors and particulate matter are exhausted from the chamber to realize a “cleaning” of the chamber. With this technique, it is not necessary to open the chamber for cleaning; thus no time-consuming machine-setup is required after cleaning.
Monitoring of such plasma cleaning processes may be accomplished through optical emission spectroscopy (OES), a technique by which light emitted by a process, such as a plasma etch within a chamber, is analyzed to see which wavelengths are present in the light. Inferences about the process may then be drawn as a result of the intensity of the various spectral lines present in the light. For example, the presence of certain species within the chamber may be ascertained because each kind of molecule or atom has a characteristic optical emission at specific wavelengths (optical emission spectrum). When the cleaning plasma reacts with the accumulated by-products, related product species are formed and may be identified by their unique and characteristic optical emission. After the accumulated by-products have been removed from the chamber and the chamber walls are exposed to the plasma, there is a change in the optical emission. This change in optical emission can be used to detect the endpoint of the cleaning process.
Optical emission spectra may be detected using a light or optical sensor. Due to the extreme environmental conditions within a chamber, the sensor device is usually placed outside the chamber. This configuration requires that the chamber have an emission detection window for the plasma light emissions to pass through before detection by a light emission spectrometer or other sensor device.
A major problem with OES monitoring encountered during a plasma clean or etch process is inconsistent or misleading emission signals due to several reasons. One cause may be the build up of contaminants on the surface of the detection window itself. This is due to a reaction product which is deposited on the surface of the window, or an unevenness which is created thereon by the impact of the ions from the plasma. As a result, when a plasma light emission passes through the window, a waveform component inherent to the material forming the deposit is absorbed or it is randomly reflected on the inner surface of the window so that the light intensity is decreased. This problem results in false readings of the state of the etch process and inefficient cleaning. Further, contaminants on the window may be getting etched by the plasma to give inconsistent readings. The plasma itself is also not in a constant state because of fluctuating power, flow rates, or other causes.
Currently, a time-mode technique is typically used for detecting the endpoint of a plasma clean. The cleaning process is performed for a fixed period of time estimated to remove the by-products accumulated on the chamber walls. Thus, it is obvious that either under-cleaning or over-cleaning is likely to occur. Under-cleaning may lead to particle problems and degradation of processing stability. Over-cleaning results in the highly reactive cleaning gases attacking the chamber wall and shortening its lifetime. Further, expensive cleaning gases may be wasted and cleaning time is not optimized.
Therefore, what is needed is a monitoring method and system which can provide an accurate indication of the endpoint of a plasma chamber cleaning process or plasma etch process without being affected by a change in plasma light transmission caused by contaminants on a detection window or changing plasma state in order to optimize the cleaning or etch process and prevent over-cleaning/etching or under-cleaning/etching.